Renewable sources of insulin-producing beta-cells for transplantation therapy of diabetes mellitus will most likely derive from the managed development of surrogate cells, such as embryonic or adult 'stem'cells. This goal would be facilitated greatly by a thorough understanding of the extrinsic morphogens and cell-autonomous factors that control embryonic pancreatic development. P48/PTF1A is a pancreas-restricted bHLH transcription factor originally thought to functional selectively in specifying acinar cell fate. Recently P48/PTF1A has been shown to be required for the formation of islet as well as acinar tissue, and mutations in the human gene cause permanent neonatal diabetes mellitus (PNDM). The functional form of P48 in pancreatic acinar cells is the heterotrimeric PTF1 complex. We have discovered that PTF1 from adult pancreas comprises P48, one of the common E proteins (E12, E47 or HEB), and SUH-L (aka RBP-Jk), a mammalian homologue of Drosophila Su(H). We have shown that the Notch-responsive mammalian orthologue of Su(H) (SUH) can also form a trimeric complex with P48 and an E protein and as well as with their evolutionarily conserved Drosophila orthologues. The human PNDM nonsense mutations delete the C-terminal region of P48, which we have shown is required for interactions with SUH and SUH-L, and therefore would not be able to form the trimeric PTF1 complex. We propose that PTF1 containing either SUH or SUH-L and in concert with Notch-signaling may control cell-fate decisions during the 'secondary transition'of pancreatic organogenesis. We will test this through three Specific Aims. Aim 1 will use a genetic approach for depleting P48 at different developmental stages in utero to determine which developmental processes require P48 and to identify direct P48-target genes. Aims 2 and 3 will determine which of the P48-dependent processes also require SUH or SUH-L and whether that requirement includes their participation in a PTF1 complex, as predicted.